1. Field of the Invention
The invention relates to a dryer section in a paper machine or in a cardboard machine or other machine for drying a moving web, especially a fibrous web.
2. Description of the Related Art
Dryer sections of this type are known in a plurality of design forms. See U.S. Pat. No. 6,365,004 B1, WO 2005/068713 A1, DE 10 2004 017 810 A1 and DE 10 2004 017 811 A1.
Known dryer sections are constructed as follows: they include a first row of dryer cylinders, and also a second row of dryer cylinders below the first row. The two rows generally progress parallel to each other and in horizontal direction. The dryer cylinders of one row are staggered relative to the dryer cylinders of the other row. All dryer cylinders are wrapped by a water vapor-permeable woven fabric belt, for example a dryer felt. The woven fabric belt represents a closed loop and rotates during the operation of the dryer section. At the same time the woven fabric belt carries the material web, for example a paper web which is to be dried. The web alternately is in direct contact with the surface area of a dryer cylinder and is enveloped by the fabric belt, while on the subsequent cylinder it lays freely on the fabric belt.
Frequently several dryer groups are provided. Viewed in direction of web travel these dryer groups are arranged in tandem. This has the advantage that a variety of influences can be exerted upon the web during the course of its travel through the entire dryer section.
The web is to be transferred from one dryer group to the subsequent one. The transfer may be accomplished such that the web is not exposed at any time and such that it is however sandwiched between the fabric belt of a first dryer group and the fabric belt of the subsequent second dryer group.
There are however also situations where the enclosed web transfer is not possible due to constructive or spatial reasons, so that the web must be transferred exposed and unsupported from one dryer group to the subsequent one. The web is subject to a certain tensile stress in longitudinal direction. In an exposed transfer one refers to exposed web segments. Exposed web segments are neuralgic locations, because of potential web breaks.
The danger of web breaks at exposed web segments depends on various factors. Papers having a high basis weight dry slower than papers which have a low basis weight. The dryer section in the paper machine however has only a certain drying capacity. This may be sufficient for papers having a low basis weight, however the tensile strength of such papers is also naturally lower.
It would therefore be desirable to adapt the drying capacity of the web to the prevailing conditions, thereby giving more consideration to different paper characteristics such as basis weight, tensile strength, etc. However, this is not possible.
What is needed in the art is to create a dryer section in a paper machine so that it is adaptable to varying requirements with regard to the web which is being processed, as well as to other influences. Exposed web segments should also be avoided. Ultimately, an existing dryer section should be able to be retrofitted accordingly.